1. Field of the Invention
The invention concerns an analog signal logarithmic envelope detector, namely a detector that gives a signal representing the slow overall variations but not the instantaneous variations of the level of an analog signal applied to its input. The signal representing the overall variations of the input signal broadly follows the contour of the maximum values of the input signal, and this is why we speak of a signal envelope detector.
However, the detector concerned herein is a logarithmic detector, that is, the overall variations of the signal are measured with a logarithmic scale: for the small levels of signal input, the variations are taken into account with a high coefficient, and the more the signal level increases, the more the variations are taken into account with a low multiplier coefficient.
By way of an example, signal logarithmic envelope detectors are useful in the analysis of speech. The signal for which it is sought to measure the envelope is the speech signal. It is thus possible to know the mean sound level of the speech at the input. A very concrete example of the use of signal envelope logarithmic detectors may be found in telephone sets with amplified listening facility. The mean level of a signal emitted on the line is compared with the level of the signal received from the line. Moreover, it is ascertained that the signal emitted or received is rather a noise signal or rather a speech signal, and action is taken on the gain of the emission and reception amplifiers, depending on the result of these determinations, so as to avoid any Larsen effect. Logarithmic envelope detectors are used to determine, at each instant, the mean level emitted or received. They have a logarithmic characteristic to improve the detection and the comparisons for the small signals without risk of saturation for the big signals.
2. Description of the Prior Art
Analog signal envelope detectors with logarithmic function are made in the prior art according to the diagram shown in FIG. 1.
They comprise essentially a logarithmic gain amplifier 10 receiving the analog signal, the level of which has to be controlled. This amplifier is followed by a full wave rectifier 12 when the input signal has positive and negative half cycles. The rectifier is followed by a smoothing RC integrator 14, the time constant of which is chosen to make the fast variations of signals disappear and to preserve the slow variations of the envelope.
The output of the detector is taken at the output of the RC integrator.
The logarithmic amplifier is an operational amplifier looped between its output and its input by two diodes in parallel, upside down with respect to each other. Since the diodes have a logarithmic current/voltage curve when they are in direct mode and since, at any instant, at least one of the diodes is in direct mode, the amplifier has a logarithmic amplification coefficient, the amplification being far greater for the small signals than for the big ones.
FIG. 4 shows, as an example, a waveform of an analog signal received at the input of the envelope detector of FIG. 1 (line A). The line B represents the logarithmically compressed signal, at output of the amplifier 10. The range of variation of the signals has been reduced. The amplitude ratio between the small signals and the big ones is considerably smaller than on the line A. The line C represents the signal at output from the rectifier 12. Finally, the line D represents the output signal of the RC integrator 14. The fast variations of the input signal have disappeared. All that remain are the slow variations representing the envelope of the input analog signal or its mean level, but with a logarithmic scale.
In the embodiment taking the form of an integrated circuit, frequently necessary when it is desired to circuit, frequently reduce costs and bulk, it is sometimes difficult to make a logarithmic envelope detector of this kind.
For, it is practically not possible to make it by means of standard MOS technologies because the way to integrate two diodes in parallel and upside down with respect to each other is not known. Now, MOS circuits are very useful for all sorts of logic functions and this, therefore, rules out the possibility of integrating, in one and the same integrated circuit, both MOS logic functions and the envelope detector which may be needed to control these logic functions.
It is true that bipolar technologies enable the integration of both the diodes upside down with respect to each other, and it is a pity to have to be restricted to a bipolar technology only because two diodes, out of thousands of elements of the integrated circuit, cannot be integrated.
Moreover, whether the technology used is MOS technology or bipolar technology, the circuit of FIG. 1 requires high-value capacitances for the smoothing, and these capacitances cannot be integrated.
To avoid the drawbacks of the prior art, the present invention proposes an analog signal logarithmic envelope detector using a logarithmic compression analog-digital coder as a basic element.
As is known, circuits commonly called "cofidecs" are being manufactured in large quantities. Cofidecs are integrated circuits designed for telephone sets. They integrate an analog-digital coding-decoding function and signal filtering functions on one and the same integrated circuit chip.
Now, the analog-digital coder of a cofidec is a logarithmic or almost logarithmic compression coder.
The idea of the present invention is to use this coder as a basic, low-cost element which can be made by means of MOS technology to create an analog signal envelope detector.
The envelope detector will be made with a circuit that is identical to a standard cofidec, of which only the coder part and not the decoder part will be used.
The analog signal for which the mean level is measured will be applied to the input of this coder. The digital output of the coder, representing the amplitude (in terms of absolute value) of the signal will be compared to the content of a counter. If the analog signal digitalized by the encoder is greater than the content of the counter, the counter is incremented, with a relatively high frequency F1. If, on the contrary, the digitalized analog signal will become smaller than the content of the counter, the counter is decremented, but with a lower frequency FR2. The content of the counter then represents a digital approximation of the envelope of the input signal.
There is no need for a rectifier as in the prior art. The standard cofidec gives a digital signal, of which one most significant bit represents the sign and the other bits represent the amplitude of the signal in terms of absolute value of the signal. It therefore suffices to compare the content of the counter with the amplitude bits of the digitalized analog signal in eliminating the sign bit.
There is no need for any RC integrator. The peak value detecting function, fulfilled in FIG. 1 by the rectifier/RC integrator unit, is fulfilled herein by the counter (the incrementation frequency of which is greater than the decrementation frequency) and the comparator which can very easily be totally integrated into a MOS technology circuit. However, the output signal from the envelope detector according to the invention is a digital signal here, and not an analog one. Most usually, this is of no importance for the final goal is to compare the level of the envelope signal with a determined value or with another signal, and these comparisons can be done on digital signals as well as on analog signals.